1. Field of the Invention
The present invention relates to a substrate for a liquid crystal display used in a display section of an information apparatus and a liquid crystal display having the same.
2. Description of the Related Art
In the field of active matrix liquid crystal displays having a thin film transistor (TFT) at each pixel, efforts have recently been made to increase their size and to allow them to display in higher tones with higher contrast.
FIG. 70 shows a configuration of one pixel of a TFT substrate of an active matrix liquid crystal display. As shown in FIG. 70, a plurality of gate bus lines 112 extending in the horizontal direction of the figure are formed substantially in parallel with each other (two of those are shown in FIG. 70). A plurality of drain bus lines 114 extending in the vertical direction of the figure are formed substantially in parallel with each other such that they intersect the gate bus lines 112 with an insulation film which is not shown interposed therebetween (two of those are shown in FIG. 70). Regions surrounded by the plurality of gate bus lines 112 and drain bus lines 114 are pixel regions. Pixel electrodes 116 are formed in the pixel regions. Storage capacitor bus lines 118 extending substantially in parallel with the gate bus lines 112 are formed substantially across the middle of the pixel regions.
TFTs 110 are formed in the vicinity of the positions where the gate bus lines 112 and the drain bus lines 114 intersect each other. Drain electrodes 122 of the TFTs 110 are extended from the drain bus lines 114 and are formed such that they are located at edges of active semiconductor layers and channel protection films formed on the same (both of which are not shown) on one side thereof. Source electrodes 124 of the TFTs 110 are formed such that they face the drain electrodes 122 with a predetermined gap left therebetween and such that they are located edges of the active semiconductor layers and channel protection films on another side thereof. Regions of the gate bus lines 112 directly under the channel protection films serve as gate electrodes of the TFTs 110. The source electrodes 124 are electrically connected to pixel electrodes 116 through contact holes (not shown).
FIG. 71 shows alignment of liquid crystal molecules in a VA (vertically aligned) mode liquid crystal display fabricated using a TFT substrate as shown in FIG. 70. The arrows in the figure represent directions in which the liquid crystal molecules are tilted when a voltage is applied to the liquid crystal layer. FIG. 71 shows three pixels which are defined by a black matrix (BM) 140. As shown in FIG. 71, the liquid crystal molecules are tilted in various directions in the VA mode liquid crystal display that has not been subjected to an aligning process such as rubbing when a voltage is applied to the liquid crystal layer. As a result, alignment regions having different areas are formed in each of the pixels. Further, boundaries (disclination) between alignment regions are visually perceived as dark lines 142 the position of which is different in each pixel. Therefore, when the display screen is viewed in a diagonal direction in particular, irregularities, coarseness, and after images can be visually perceived, and image quality can be thus significantly reduced.
Liquid crystal displays are now being used even as monitors of personal computers (PC) and television receivers. In such applications, liquid crystal displays must have wider viewing angles such that they can be properly viewed in any direction.
MVA (multi-domain vertical alignment) type liquid crystal displays (hereinafter referred to as “MVA LCDs”) have been proposed as a technique to achieve wider viewing angles (see Article 1, for example).
FIGS. 72A and 72B show a schematic sectional configuration of an MVA LCD. FIG. 72A a liquid crystal layer to which no voltage is applied, and FIG. 72B shows the liquid crystal layer to which a predetermined voltage is now applied. As shown in FIGS. 72A and 72B, the MVA LCD has two substrates 302 and 304 which are provided opposite to each other. A transparent electrode (not shown) is formed on both of the substrates 302 and 304. A plurality of linear protrusions (banks) 306 made of resin are formed in parallel with each other on the substrate 302, and a plurality of linear protrusions 308 are formed in parallel with each other on the other substrate 304. The protrusions 306 and 308 are alternately arranged when viewed in a direction perpendicular to substrate surfaces.
A liquid crystal layer 160 having negative dielectric constant anisotropy is sealed between the substrates 302 and 304. As shown in FIG. 72A, liquid crystal molecules 312 are aligned substantially perpendicularly to the substrate surfaces by an alignment regulating force of a vertical alignment film (not shown) formed on surfaces of the substrates 302 and 304 facing each other. Liquid crystal molecules 312 in the vicinity of the protrusions 306 and 308 are aligned substantially perpendicular to inclined surfaces of the protrusions 306 and 308. That is, the liquid crystal molecules 312 in the vicinity of the protrusions 306 and 308 are aligned at an angle to the substrate surfaces.
As shown in FIG. 72B, when a predetermined voltage is applied between the transparent electrodes on the substrates 302 and 304, the liquid crystal molecules 312 in the vicinity of the protrusions 306 and 308 are tilted in a direction perpendicular to the direction in which the protrusions 306 and 308 extend. The tilt is propagated to liquid crystal molecules 312 between the protrusions 306 and 308, and the liquid crystal molecules 312 between the protrusions 306 and 308 are thus tilted in the same direction.
The tilting direction of the liquid crystal molecules 312 can be regulated in each region by providing the protrusions 306 and 308 in such a way. When the protrusions 306 and 308 are formed in two directions substantially perpendicular to each other, the liquid crystal molecules 312 are tilted in four directions in one pixel. Since viewing angle characteristics of different regions are thus mixed, the MVA LCD has a wide viewing angle when displaying white or black. The MVA LCD exhibits a contrast ratio of 10 or more in upward, downward, leftward, and rightward viewing directions each of which is at an angle of 80 deg. to the direction perpendicular to the display screen.
(Reference Documents)
Article 1: Japanese Patent No. 2947350
Article 2: JP-A-2000-305100
Article 3: JP-A-2001-249340
Article 4: JP-A-2001-249350
Article 5: JP-A-2002-40432
Article 6: JP-A-2002-40457
Article 7: JP-A-2000-47251
However, the MVA LCD shown in FIGS. 72A and 72B has a problem arises in that it suffers from a low yield of manufacture and a high manufacturing cost because there is a need for an additional step for forming the protrusions 306 and 308.
Another technique is to form a transparent electrode with blank sections (slits) instead of providing protrusions 306 and 308. However, when a common electrode on a CF substrate is formed with slits, a CF layer is exposed and put in contact with a liquid crystal layer. For example, when resin including a pigment dispersed therein as a color component is used as a CF layer, a problem arises in that inorganic components of the pigment can contaminate a liquid crystal layer and a semiconductor layer.
FIG. 73 shows another configuration of a TFT substrate of an MVA LCD. As shown in FIG. 73, a pixel electrode 116 has trunk sections 128 extending substantially in parallel with or perpendicularly to bus lines 112 and 114, branch sections 130 branching from the trunk sections 128 and extending diagonally to the same, and spaces 132 between adjoining branch sections 130. In an MVA LCD fabricated using the TFT substrate shown in FIG. 73, the aligning direction of liquid crystal molecules is determined by the trunk sections 128 and the branch sections 130.
However, since the response time of the liquid crystal molecules in the MVA LCD fabricated using the TFT substrate shown in FIG. 73 is long, a singular point of an alignment vector of liquid crystal molecules is generated at random on the branch sections 130. As a result, the position of a singular point is different in each pixel or frame. Therefore, irregularities and coarseness is visually perceived on the display screen when the display screen is viewed in a diagonal direction in particular, which results in the problem of reduction in display quality.